Phylogenetics and chromosomal evolution in the Poaceae (grasses)

Transcription

1 CSIRO PUBLISHING Australian Journal of Botany, 2004, 52, Phylogenetics and chromosomal evolution in the Poaceae (grasses) Khidir W. Hilu Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia , USA. Abstract. The wide range in basic chromosome number (x = 2 ) and prevalence of polyploidy and hybridisation have resulted in contrasting views on chromosomal evolution in Poaceae. This study uses information on grass chromosome number and a consensus phylogeny to determine patterns of chromosomal evolution in the family. A chromosomal parsimony hypothesis is proposed that underscores (1) the evolution of the Joinvilleaceae/Ecdeiocoleaceae/Poaceae lineage from Restionaceae ancestors with x = 9, (2) aneuploid origin of x = 11 in Ecdeiocoleaceae and Poaceae (Streptochaeta, Anomochlooideae), (3) reduction to x = 9, followed by chromosome doubling within Anomochlooideae to generate the x = in Anomochloa, and (4) aneuploid increase from the ancestral x = 11 to x = in Pharoideae and Puelioideae, and further diversification in remaining taxa (Fig. 3b). Higher basic chromosome numbers are maintained in basal taxa of all grass subfamilies, whereas smaller numbers are found in terminal species. This finding refutes the secondary polyploidy hypothesis, but partially supports the reduction hypothesis previously proposed for chromosomal evolution in the Poaceae. BT03103 Chromosome K. W. Hilu evolutionin Poaceae Introduction Variation in basic chromosome number, high incidence of polyploidy, frequent hybridisation and wide range of variation in genomic size are prominent features of grass-genome evolution. These features are evident in the estimated 80% polyploidy, basic chromosome numbers of x = 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,, 13, 14 and (dewet 1987; Hunziker and Stebbins 1987), and 2C DNA contents of 0.7 in Chloris gayana to 27.6 in Lygeum spartum (Bennett et al. 2001). The distribution of these basic chromosome numbers in the various grass subfamilies has resulted in wide disagreements on the polarity and trends of chromosome evolution. High frequency of polyploidy, particularly allopolyploidy and aneuploidy, a tendency for rediploidisation and frequent hybridisation between closely and distantly related species compound the difficulties in deducing the trends. Above all, lack of a robust phylogeny based on empirical approaches has resulted in equivocal assertions on chromosomal evolution in Poaceae. Consequently, contrasting views have emerged on chromosomal evolution in the family (Avdulov 1931; Flovik 1938; Hubbard 1948; Raven 1975; Clayton 1981; Stebbins 1982, 1985; dewet 1987; Hunziker and Stebbins 1987). Currently, a general consensus on grass phylogeny does exist and the identity of the most basal lineages is well documented, making prediction and interpretation of chromosomal evolution more attainable. In this study, available knowledge of chromosome variation and its patterns of distribution in grass subfamilies and tribes will be evaluated and superimposed on a well supported consensus phylogeny. Previous hypotheses on chromosomal evolution in Poaceae will be critically discussed and a new assessment will be formulated. Information on chromosome numbers is obtained from Dahlgren et al. (1985), Clayton and Renvoize (1986), Watson and Dallwitz (1992), Kubitzki (1998), and several other references cited here. Current hypotheses on chromosomal evolution in Poaceae The ancestral chromosome number and trends in chromosomal evolution in grasses have been the subject of considerable speculation. Avdulov (1931) proposed the reduction hypothesis which maintains that x = is the ancestral number from which lower numbers originated through aneuploidy. Raven (1975) agreed with this hypothesis and used as evidence the prevalence of x = in the Bambusoideae, which were presumed to be the most ancestral grasses. He concluded that x = 6, found in some advanced genera, is of aneuploid origin. Stebbins (1982, 1985) disagreed with the reduction hypothesis and presented the secondary polyploidy hypothesis which suggests x = and 11 as secondary chromosome numbers CSIRO /BT /04/010013

2 14 Australian Journal of Botany K. W. Hilu derived from x = 5 and 6. He used the countervailing arguments that x = 6 and 7 are found in the most primitive genera of their tribes, such as Danthonia and Pentaschistis of the Danthonieae and Sorghum of the Andropogoneae. Hubbard (1948), Stebbins (1985) and dewet (1987) proposed independent origins for the basic chromosome numbers in the major groups of grasses from species complexes with x = 5, 6 and 7. DeWet (1987) argued that if one followed Clayton s (1981) proposed scheme of grass evolution, the Arundinoideae with its unspecialised genera and broad diversity in basic chromosome numbers (x = 6, 7, 9, 10, 11) would have given rise to the x = 7 of the Pooideae, x = 9 and 10 of the Panicoideae and Chloridoideae, whereas x = 10 of the Bambusoideae and x = of Oryzoideae and Centothecoideae would be derived from a pre-arundinoid grasses. This evolutionary pattern was not convincing to him and, thus, he speculated that the major groups of Poaceae were independently derived from an original species complex with x = 5, 6 and 7. He asserted that hybridisation and chromosome doubling, followed by diploidisation at the tetraploid level, gave rise to secondary diploids, with x = 10, and 14. DeWet also indicated that aneuploidy and further cycles of polyploidy resulted in somatic chromosome numbers ranging from 2n = 6 to 2n = 265. Mehra et al. (1968), Sharma (1979) and Hunziker and Stebbins (1987) argued in favour of x = 6 as the most ancestral chromosome number. Hunziker and Stebbins (1987) speculated that from this number, the secondary basic number of x = originated and subsequently gave rise to higher polyploids. Aneuploid events in both directions gave origin to x = 7 and 5. They also proposed that x = 5 is ancestral to x = 10, and from the latter evolved x = 11, 9, 8, and again 7 (as in Olyra). Patterns of variation in Poaceae chromosome numbers The basic chromosome numbers of x = 2 in the Poaceae have generated somatic numbers that vary between 2n = 4 and 2n = (dewet 1987). The most common basic chromosome numbers in the family are 7, 9, 10 and (Stebbins 1982). The number x = 2 is reported for the Pooideae species Colpodium versicolor (tribe Poeae) and Zingeria biebersteiniana (tribe Aveneae) (Tsvelev and Zhukova 1974; Sokolovskaya and Probatova 1977). Colpodium contains species with x = 2, 4, 5, 6, 7 and 9, and Zingeria species with x = 2, 4 and 6; both series may represent aneuploid reduction derived from the pleisomorphic number x = 7 that is predominant in these tribes. These chromosomal series underscore the remarkable flexibility in genomic evolution in the Poaceae. Various patterns of variation at the subfamily level are detectable (Fig. 1). The subfamily Anomochlooideae, encompassing Anomochloa and Streptochaeta, is based on x = and 11, respectively (Pohl and Davidse 1971; Davidse and Pohl 1972; Hunziker and Stebbins 1987; Hunziker et al. 1989). The Pharoideae have x = (Davidse and Pohl 1972; 13,,11, 9, 8, , 11, 11, 10, (7), (17, 15, 10) 13, 11, or 11, 9-2 JOINVILLEACEAE ECDEIOCOLEACEAE Streptochaeta Anomochloa Pharoideae Puelioideae Bambusoideae Ehrhartoideae-core Ehrhartoideae-basal Pooideae-core Pooideae-basal PACCAD-core PACCAD-basal Clayton and Renvoize 1986). In the Bambusoideae s.s., the basic number x = is characteristic of woody members, although a few species with x = 10 have been reported (Clayton and Renvoize 1986). Herbaceous members of the Bambusoideae, however, have basic numbers of x = 7, 10, 11 and (Tateoka 1955; Kammacher et al. 1973; Hunziker et al. 1982; Clayton and Renvoize 1986; Hunziker and Stebbins 1987). Homogeneity in chromosome number is a trademark of the oryzoid grasses, with x = appearing in all but Zizania and Microlaena. In contrast, the Pooideae stands out with the highest diversity in chromosome number, displaying a wide range of numbers from x = 2 to x = 13. Diversity in basic chromosome number is also evident in the Arundinoideae (x = 6, 9, ), Panicoideae (x = 5, 7, 9, 10,, 14) and Chloridoideae (x = 7, 8, 9, 10). However, these subfamilies tend to have predominant basic numbers (such as x = 9 and 10 in the Panicoideae and Chloridoideae) and some rare numbers. Current views on grass evolution Cladistic analyses of morphological, anatomical and molecular information in the past two decades have refined our understanding of grass phylogeny (reviewed in Hilu et al. 1999; GPWG 2001) and a consensus phylogeny has emerged (Fig. 1). Recent molecular studies in particular have provided critical testing of the various proposed hypotheses on grass evolution. A general agreement on the monophyly of subfamilies and tribes and their phylogenetic position are evident (Barker et al. 1995; Clark et al. 1995; Hilu et al. 1999; GPWG 2001). Owing to the high congruence between the two recent comprehensive treatments of the family by 9-2 RESTIONACEAE ANARTHRIACEAE Fig. 1. A consensus tree for the Poaceae based on Hilu et al. (1999) and GPWG (2001) molecular and non-molecular data. Basic chromosome numbers are mapped on the tree. Dark lines represent potentially unresolved lineages and parentheses enclose infrequent chromosome numbers in taxa.

3 Chromosome evolution in Poaceae Australian Journal of Botany (11) 11 (), 10, , 6, 7, 9 6, 9, Micraira Centothecoideae Gynerieae core Panicoideae Panicoideae Eriachne Aristideae Danthonioideae Arundinoideae s.s. to BEP because the name Oryzoideae is replaced by Ehrhartoideae). Although the alliance among these clades is not strongly supported, they are phylogenetically distinct from the PACCAD subfamilies (Fig. 1). Another important finding that will also have an impact on interpreting chromosome evolution in grasses is the solidification of the phylogenetic positions of some tribes whose affinities have been uncertain. Outstanding among these are the Diarrheneae, Ehrharteae, Lygeeae, Micraireae, Nardeae and Stipeae. It is also to be noted that the identities of basal lineages in a number of subfamilies are well ascertained and the phylogenetic positions of various tribes are well resolved (Hilu et al. 1999; GPWG 2001). (), 10, 9 Merxmuellera Centropodia Chloridoideae Fig. 2. A consensus tree of the PACCAD clade based on Hilu et al. (1999) and GPWG (2001), with basic chromosome numbers mapped on the tree. Solid triangles represent core groups in the lineages, lines depict basal taxa and parentheses enclose infrequent chromosome numbers in taxa. using matk data (Hilu et al. 1999) and molecular and structural information (GPWG 2001), this paper will follow the taxonomic treatment proposed by the latter study. The Anomochlooideae (Anomochloa and Streptochaeta) is a sister taxon to remaining Poaceae; diverging next in a grade are the Pharoideae, Puelioideae, and then the remaining grasses (Barker et al. 1995; Clark et al. 1995; Soreng and Davis 1998; Hilu et al. 1999; GPWG 2001). Although the monophyly of Anomochloa and Streptochaeta has been questioned (Hilu and Alice 1999; Hilu et al. 1999, Zhang 2000), their most basal position in the phylogeny is not. Further support for the basal lineages is evident in the lack of a 1-bp deletion that caused a frame shift at the 3 end of the matk gene in all grasses but Streptochaeta and Anomochloa (Hilu and Alice 1999). The lack of this mutation is shared with the outgroup Joinvilleaceae but not Restionaceae (Hilu and Alice 1999). The subfamilies Arundinoideae, Centothecoideae, Chloridoideae and Panicoideae form a strongly supported lineage named PACC (Esen and Hilu 1989; Davis and Soreng 1993; Barker et al. 1995; Clark et al. 1995; Hilu and Alice 1999; Hilu et al. 1999). Recently, subfamilies Aristidoideae and Danthonioideae have been recognised to accommodate the isolated tribe Aristideae and a danthonioid segregate from the Arundinoideae, respectively, modifying the acronym to PACCAD (Fig. 2). The following two major groups are evident in the PACCAD lineage: (1) panicoid and centothecoid, and (2) aristidoid, danthonoid, arundinoid and chloridoid (GPWG 2001). In addition to the PACCAD, Clark et al. (1995) recognised the BOP lineage that comprises bambusoid, oryzoid and pooid taxa (the acronym is changed Sister-group relationships of Poaceae Another issue crucial to understanding chromosomal evolution in the Poaceae is the status of the sister-group relationship to the family. Poales as defined by Dahlgren et al. (1985) and confirmed in recent studies (see Briggs et al and Michelangeli et al. 2003) comprises the families Flagellariaceae, Centrolepidaceae, Anarthriaceae, Restionaceae, Joinvilleaceae, Ecdeiocoleaceae and Poaceae. Flagellariaceae is generally considered as sister to remaining Poales (Briggs et al. 2000; Linder et al. 2000; Michelangeli et al. 2003). The precise position of Anarthriaceae and Centrolepidaceae in Poales has been debated; however, the two families are closer to the Restionaceae than Joinvilleaceae, Ecdeiocoleaceae and Poaceae (Briggs et al. 2000; Linder et al. 2000; Bremer 2002; Michelangeli et al. 2003). Bremer (2002) and Michelangeli et al. (2003) showed Anarthriaceae as sister to Restionanceae. Among Poales families, it has been demonstrated that Joinvilleaceae and Ecdeiocoleaceae are most closely related to the Poaceae (Doyle et al. 1992; Kellogg and Linder 1995; Hilu and Alice 1999; Briggs et al. 2000; Bremer 2002; Michelangeli et al. 2003). This assessment is based on analyses of morphological and molecular data. In particular, two inversions found in the plastid genome of the Poaceae (Hiratsuka et al. 1989; Shimada and Sugiura 1991; Doyle et al. 1992; Katayama and Ogihara 1996; Michelangeli et al. 2003) have been effective in ascertaining phylogenetic relationships in Poales. These inversions are called the 28-kilobase (kb) and 6-kb inversions (a third inversion, called trnt inversion, is confined to Poaceae). Outside the Poaceae, the 28-kb inversion was detected only in the plastid genomes of the Joinvilleaceae, Ecdeiocoleaceae and some Restionaceae (Doyle et al. 1992; Katayama and Ogihara 1996; Michelangeli et al. 2003). Doyle et al. (1992) found the 6-kb inversion in Joinvilleaceae and Restionaceae, but were unable to detect it in the Ecdeiocoleaceae. However, the presence of the 6-kb inversion in the Ecdeiocoleaceae was recently demonstrated by Michelangeli et al. (2003). Therefore, the 28-kb inversion defines the Restionaceae, Joinvilleaceae, Ecdeiocoleaceae and Poaceae clade in Poales, whereas the 6-kb inversion is a

4 16 Australian Journal of Botany K. W. Hilu synapomorphic character uniting Joinvilleaceae, Ecdeiocoleaceae and Poaceae (designated here as JEP). Hilu et al. (1999, 2003), using matk sequence data, resolved Joinvilleaceae as sister to Poaceae, with Restionaceae being sister to both. This sister-group relationship was further confirmed by identifying base substitutions and an insertion at the 3 end of the plastid matk gene that resulted in a longer reading frame in these two families. Restionaceae lacked that indel but emerged as sister to both. Ecdeiocoleaceae was not examined in these studies. Insight into the phylogenetic relationship among the JEP families comes from recent analyses of molecular and morphological information (Briggs et al. 2000; Bremer 2002; Michelangeli et al. 2003). Analysis of combined sequence data from rbcl and trnt-f regions (Briggs et al. 2000) resolved Ecdeiocoleaceae and Poaceae in a clade with 99% jackknife (JK) support (trnt-f data were not available for Joinvilleaceae); rbcl data alone showed Joinvilleaceae + Ecdeiocoleaceae sister to Poaceae (no jackknife values were reported). However, strongest support for the sister-group relationship of Ecdeiocoleaceae and Poaceae was evident in a parsimony and Bayesian analyses of sequence information from rbcl and atpb (Bremer et al. 2002) where the two families formed a clade supported by 99% JK and 1.00 posterior probability; Joinvilleaceae appeared as the sister to this clade (100% JK and 1.00 posterior probability). A similar topology emerged in an analysis of combined structural data and sequence information from rbcl and atpa (Michelangeli et al. 2003). Although the Ecdeiocoleaceae/Poaceae clade received low jackknife support (49%), the two alternative topologies, Joinvilleacea/Poaceae and Ecdeiocoleaceae/Joinvilleacea, were found to be least likely (<20% jackknife support). The basic chromosome numbers in the families concerned are the following: Flagellariaceae, x = 19 (Appel and Bayer 1998); Anarthriaceae, x = 9 (Linder et al. 1998); and Joinvilleaceae, x = (Bayer and Appel 1998). In Restionaceae, basic numbers of x = 6, 7, 9, 11, and 13 have been reported for Australian taxa (Linder et al. 1998) and 2n = 16 and 20 for African taxa (Krupko 1962, 1966). Chromosome numbers in Ecdeiocoleaceae are not precise as chromosome counts of 2n = for Georgeantha and 2n = ~48 for Ecdeiocolea have been reported (B. Briggs, unpubl., cf. Linder et al. 1998). Centrolepidaceae is poorly known, with reports of n = 10 and 13 in two Centrolepis species, and n = 20 and ~24 in another two (Goldblatt 1980), implying that x = 10 for the former and x =, or most likely 13, for the latter. Results Trends in chromosome evolution To establish a robust theory on chromosomal evolution of any taxonomic group, three crucial points have to be unequivocally determined: (1) monophyly of the ingroup, (2) the identity of the sister group, and (3) a robust phylogeny, with most basal lineages well defined. The monophyly of the Poaceae has been well ascertained in several studies (Barker et al. 1995; Clark et al. 1995; Duvall and Morton 1996; Liang and Hilu 1996; Soreng and Davis 1998; Hilu et al. 1999; GPWG 2001). The sister-group relationship of Poaceae to Ecdeiocoleaceae and Joinvilleaceae is well documented and is detailed above. In terms of the identity of most basal lineages in the Poaceae, the subfamilies Anomochlooideae and Pharoideae, respectively, are the first taxa to diverge in phylogenies, based on different types of molecular and non-molecular data (Clark et al. 1995; Hilu and Alice 1999; Hilu et al. 1999; GPWG 2001). Therefore, all three basic criteria for a critical assessment of chromosome evolution for the grass family are met. Chromosomal evolution at the base of grasses The basic chromosome number in the Joinvilleaceae and Anomochloa is x =. Such a number is generally considered to be a rediploidised ancient polyploid (Masterson 1994) and, therefore, it is derived from x = 9. Streptochaeta is based on x = 11. The chromosome counts in Ecdeiocoleaceae of 2n = and ~48 are most likely based on x = 11 (hexaploid) and x = (tetraploid), respectively. Assuming a basic chromosome number of x = 9 for the Ecdeiocoleaceae would imply heptaploidy for 2n = and pentaploidy for x = ~48; both are unlikely as they would require either overcoming meiotic irregularities or acquiring consistent apomixis. Considering the chromosome numbers reported for the Restionaceae and the sister-group relationship of this family to the JEP clade, both x = 9 and x = 11 have to be considered in assessing chromosomal evolution from Joinvilleacea to Ecdeiocoleaceae and Poaceae. On the basis of these numbers, two hypotheses are proposed and illustrated in Fig. 3a, b. In both hypotheses, aneuploidy at x = 9 and 11 but not at x = is assumed, and x = in Joinvilleaceae and Anomochloa has arisen independently by chromosome doubling of x = 9. If the parsimony principal is invoked here, the choice between these two chromosomal hypotheses (x = 9 v. x = 11) will depend on ascertaining the ancestral chromosome number for the Anomochlooideae (x = 9 or 11) should the subfamily be considered monophyletic or whether Streptochaeta (x = 11) or Anomochloa (x = ) is the sister taxon to remaining Poaceae should paraphyly be assumed for the subfamily. Information on this point may come from the three cases where the Anomochlooideae appeared paraphyletic; in all these cases, Streptochaeta emerged sister to Anomochloa plus remaining Poaceae (Hilu et al. 1999; Hilu and Alice 1999; Zhang 2000). Considering the ancestral chromosome number in Anomochlooideae, and consequently Poaceae, as x = 11, the hypothesis depicting x = 9 as ancestral basic chromosome number for the JEP

5 Chromosome evolution in Poaceae Australian Journal of Botany 17 (a) Ecdeiocoleaceae (11,!) Joinvilleaceae () (b) Ecdeiocoleaceae (11,!) Joinvilleaceae () Streptochaeta (11) 9 Restionaceae ancestors X = 11 Streptochaeta (11) 11 Restionaceae ancestors X = 9 11 Pharus () Anomochloa () 9 Puelia () lineage is more parsimonious, with Joinvilleaceae sister to Ecdeiocoleaceae plus Poaceae. In this case, only one aneuploidy event from x = 9 to x = 11 is required to the base of the grass tree (Fig. 3). In contrast, aneuploidy from x = 11 to x = 9 and a reversal to x = 11 is required should one assume x = 11 as an ancestral number for JEP. Further support for a chromosomal evolution from x = 9 in Joinvilleaceae to x = 11 in Ecdeiocoleaceae and Streptochaeta in Poaceae comes from the position of Anarthriaceae (x = 9) in Poales. Bremer (2002) showed Anarthriaceae and Restionaceae as sister taxa (93% JK and 1.0 posterior probability). In the molecular structural study of Michelangeli et al. (2003), Anarthriaceae emerged sister to Centrolepidaceae plus Restionaceae (87% JK). Similar topologies were also evident in the molecular study of Briggs et al. (2000) and were inferred from flavonoid data (Williams et al. 1997). Considering the sister-group relationship of Restionaceae/Anarthriaceae to the JEP clade and the basic chromosome number of x = 9 in Anarthriaceae and x = in POACEAE Puelia () Pharus () Anomochloa () 9 POACEAE Remaining Poaceae -2 Remaining Poaceae -2 Fig. 3. Diagrammatic representations of two proposed hypotheses for chromosomal evolution from Restionaceae to Poaceae, with x = 11 (Fig. 3a) and x = 9 (Fig. 3b) as ancestral basic chromosome numbers. Changes in chromosome number are mapped on the trees. Joinvilleaceae, the JEP and the Anarthriaceae/Restionaceae clades may share the ancestral basic chromosome number of x = 9. I propose the chromosomal parsimony hypothesis to explain chromosomal evolution in the Poaceae. This hypothesis asserts that (1) the ancestral chromosome number for the Poaceae is x = 11, derived from a common ancestor with Ecdeiocoleaceae, (2) aneuploidy led to x = 9 and then chromosome doubling to x = in Anomochloa, followed by (3) an aneuploid increase from x = 11 to x = of Pharus and Puelia (Fig. 3b). This proposed theory is congruent with current views on grass phylogeny (Fig. 1), regardless of the definition of the Anomochlooideae. Chromosomal evolution of terminal grass lineages Next to diverge after these basal lineages are the BEP and PACCAD groups (Fig. 1). The former includes the Bambusoideae, Ehrhartoideae and Pooideae as currently defined (Hilu et al. 1999; GPWG 2001). Although monophyly of the group is not well substantiated and its topology is inconsistently reported in different studies, it is significant to note that high basic chromosome numbers are predominant in its subfamilies or at least their basal taxa. The number x = prevails in woody Bambusoideae and is found in all ehrhartoid grasses except for Zizania (x = 15 and 17). Herbaceous bambusoids also have high chromosome numbers (x = 10, 11 and ); the exception here is the Olyreae in which there is an aneuploid series of x = 7 and 9, in addition to x = 10 (Hunziker et al. 1982). Moreover, chromosome numbers of x = 10, 11 and 13 are characteristic of basal genera and tribes of the Pooideae (Fig. 1). Therefore, high ( 10) basic chromosome numbers remained prevalent in the family during its early stages of evolution. Only in the Pooideae, the second largest grass subfamily, did smaller chromosome numbers (x = 2 7) appear later in the subfamily (Fig. 1), probably in concert with accelerated speciation and radiation into new habitats. Within the PACCAD clade (Fig. 2), Micraireae is considered most basal (GPWG 2001). This Australian monogeneric tribe of 13 species possesses a basic chromosome number of x = 10 (Clayton and Renvoize 1986). Eriachne appears basal to one of the two PACCAD lineages that include the Aristidoideae, Arundinoideae, Danthonioideae and Chloridoideae. This mainly Australian genus also has x = 10 (Clayton and Renvoize 1986). Therefore, it is possible that the ancestral basic number for the PACCAD lineage is x = 10. This number is quite prevalent among members of the PACCAD group. It is, however, lacking in the basal Anomochlooideae, Pharoideae and Puelioideae lineages and is quite uncommon and evidently derived in the Bambusoideae. Therefore, a chromosomal link between the PACCAD clade and the basal lineages is not evident, and the x = 10 appears to have emerged in the PACCAD via aneuploidy from x =.

6 Australian Journal of Botany K. W. Hilu Chromosome evolution within subfamilies Basal subfamilies Among the basal-grade subfamilies, the Anomochlooideae has x = 11 and. In the Pharoideae, Davidse and Pohl (1972) reported x = for Pharus, and Clayton and Renvoize (1986) cited x = for the Phareae, stating that four species have been counted for three genera of the Phareae, without identifying those genera. For the Puelioideae, a subfamily comprising Puelia and Guaduella and sister to remaining grasses, the basic chromosome number x = has been reported for Puelia (GPWG 2001). Therefore, except for the disputable Anomochlooideae, homogeneity in basic chromosome number within subfamilies appears to be the norm at the base of the Poaceae. Subfamily Bambusoideae In the Bambusoideae s.s., x = is characteristic of woody Bambuseae, although a few species with x = 10 have been reported (Clayton and Renvoize 1986; Pohl and Clark 1992). Diploidy is rare in this group and polyploidy is the norm (Pohl and Clark 1992). Herbaceous members of the Bambusoideae, however, have basic numbers of x = 10, 11 and (Hunziker et al. 1982; Clayton and Renvoize 1986; Hunziker and Stebbins 1987). The Olyreae have x = 11 for 16 species, x = 10 for four, and x = 9 and 7 for one species each (Hunziker et al. 1982; Clayton and Renvoize 1986; Hunziker and Stebbins 1987). Other herbaceous bambusoids are Parianeae (two genera and ~40 species) that have x = 11 and (Clayton and Renvoize 1986) and Phaenospermateae that possess x = (Kammacher et al. 1973). Therefore, x = 11 or x = are ancestral chromosome numbers in the Bambusoideae, and lower aneuploidy series in the Olyreae and other herbaceous bambusoids are secondarily derived. This hypothesis is phylogenetically sound, considering the pattern of divergence and basic chromosome number of the basal grade in the Poaceae (x =, 11 and ; Fig. 1). The burst in chromosomal evolution in the Olyreae, the largest of the herbaceous bambusoid tribes, could be correlated with their shorter life cycle than for woody species that reproduce sexually only once every several years. Shorter life cycle allows for more frequent opportunities of meiotic recombination and subsequent possibilities for aneuploidy. What also sets the Olyreae aside from other herbaceous bambusoids is its extensive species and habitat diversity. Clayton and Renvoize (1986) indicated that the tribe not only successfully occupies the special environment of the forest floor, but seems to be undergoing adaptive radiation in that habitat. Chromosomal repatterning through aneuploidy, coupled with increased genetic heterozygosity, could provide the genetic means for radiation and habitat exploitation. Subfamily Ehrhartoideae This relatively small subfamily includes genera, occupying primarily aquatic or wet habitats. It includes the core tribes Oryzeae and Zizanieae and the basal tribes Streptogyneae and Ehrharteae. The subfamily consistently maintained x = from basal to terminal taxa, with the exception of the monoecious Zizania with its aneuploidy-derived x = 15 and 17 (Clayton and Renvoize 1986; Watson and Dallwitz 1992). The increased basic number in Zizania could not be attributed to floret unisexuality and its obligate outbreeding because Zizaniopsis and Luziola have a similar reproductive system but maintain x =. Homogeneity in basic chromosome number in the oryzoid grasses could be a consequence of the buffering effect of their aquatic habitat. Subfamily Pooideae The Pooideae is remarkable among grass subfamilies in its chromosome-number diversity. It is one of the major grass subfamilies, occupying temperate regions worldwide. Recent studies have placed the Brachyelytreae, Lygeeae and Nardeae as first diverging Pooideae, and the Meliceae, Diarrheneae and Stipeae near the base (Catalan et al. 1997; Hilu et al. 1999; GPWG 2001). Brachyelytreae, a monotypic tribe, has 2n = 22 (Tateoka 1955). The monogeneric Diarrheneae possesses 2n = 38 and 60 (Macfarlane and Watson 1980; Clayton and Renvoize 1986), and is possibly based on x = 10. A basic chromosome number of x = 10 has been reported for the only species of the Lygeeae (Clayton and Renvoize 1986). Nardus has 2n = 26 30, and may be based on x = 13 (Watson and Dallwitz 1992). The Stipeae has an extensive aneuploid series (2n = 22 66) that may be based on x = 11 (Clayton and Renvoize 1986). In the Meliceae, Melica has x = 9, whereas Glyceria and Anthoxanthum have x = 10 (dewet 1987). In the monotypic Brylkinieae, a tribe allied to the Meliceae, 2n = 40 was reported for Brylkinia (Clayton and Renvoize 1986) and is probably based on x = 10. Brachyelytrum alone (GPWG 2001), or in association with Nardus (Hilu et al. 1999), forms the most basal clade in the Pooideae. Therefore, x = 11 or higher (possibly 13) is ancestral in the Pooideae, from which x = 2 10 were derived higher in the tree (Fig. 1). The Brachypodieae represents the first lineage of the terminal pooid taxa that has undergone diversification in chromosome number. Brachypodium contains species with basic chromosome numbers of x = 5, 7 and 9 (Clayton and Renvoize 1986). Next to diverge are the Triticeae (wheat tribe) and Bromeae; both maintain a remarkable consistency of x = 7 but achieved diversity via hybridisation and polyploidy. It is not until the Poeae and Aveneae that chromosomal diversity reaches its peak with x = 2, 4, 5, 6, 7, 8 and 9. The Aveneae, although characteristically having x = 7, possesses genera in which all species have x = 5 (Briza and Anthoxanthum) or x = 4 (Airopsis, Holcus and Periballia), and others where only some of their species deviate, such as the case of x = 8 found in three species of Phalaris, in one species of Sphenopus (S. divaricatus) and

7 Chromosome evolution in Poaceae Australian Journal of Botany 19 two species of Catapodium (Hunziker and Stebbins 1987). In Milium (Aveneae), x = 4, 5, 7 and 9 have been cited (Watson and Dallwitz 1992); the perennials have x = 7, whereas the annuals have x = 5 and 4 (Stebbins 1982; Hunziker and Stebbins 1987; Watson and Dallwitz 1992). This suggests that the annuals and their chromosome numbers are derived from the perennials. It may not be coincidental that in the pooid lineage the basal taxa are mostly homogeneous in basic chromosome number and are not speciose, whereas increase in the diversity of chromosome numbers in the terminal taxa is accompanied by explosive speciation. This parallelism in chromosome number and species diversity is also evident in the pooid Stipa, a genus with ~300 species, where basic chromosome numbers of x = 9, 10, 11 and were reported along with somatic numbers of 2n = (Watson and Dallwitz 1992). The chromosomal diversity in Stipa represents speciation enhanced by various levels of euploidy and aneuploidy. Subfamily Centothecoideae This subfamily encompasses 10 genera that characteristically have x =. The exception is Zeugites where 2n = 46 is cited (Watson and Dallwitz 1992); the latter number, if correctly counted, is probably an aneuploidy, derivative of x =. The monotypic Thysanolaeneae, a tribe that used to be allocated to the Arundinoideae, has been reported to have x = 11 or x = (Watson and Dallwitz 1992). Therefore, x = is the hallmark of this subfamily. Subfamily Panicoideae The monotypic Gynerieae (x = 11) is most basal in the Panicoideae (GPWG 2001). The Arundinelleae, a basal tribe in the Panicoideae (Hilu et al. 1999; GPWG 2001), has x =9, 10, and, with x = 9 being least common (Phipps and Mahon 1970; Kammacher et al. 1973). Among the smaller Panicoideae tribes, a chromosome count is available only for the Isachneae (x = 10; Tateoka 1962). In the core Panicoideae, x = 9 and x = 10 predominate. DeWet (1987) pointed out a striking pattern of basic chromosome numbers in the two major tribes, Paniceae and Andropogoneae. The former contains x = 10 in 21% of the species and x = 9 in most of the remaining taxa, whereas the exact opposite is true for the Andropogoneae, with 85% of the species having x = 10, compared with the less common x = 9. Basic chromosome number of x = 5 is found in Parasorghum, Stiposorghum, Thelopogon, Sorghum, Zea and Elionurus (dewet 1987; Hunziker and Stebbins 1987; Melak et al. 1993). Giussani et al. (2001) have resolved in a ndhf-based tree three major clades with largely identical basic chromosome numbers, one clade representing Paniceae members with x = 9, whereas the other two comprise Andropogoneae and Paniceae with x =10 (latter two appearing as weakly supported sister lineages). Therefore, x = 11 appears as the ancestral basic number for the Panicoideae from which x = 9, 10 and other numbers emerged. Considering the sister relationship of Micraira to remaining PACCAD grasses (Fig. 2), x = 10 should theoretically be regarded as an ancestral number in this lineage. This suggests that x = 11 and of the Centothecoideae and x = 11 of Gynerium are derived, and that x = 10 of the Panicoideae represents a reversal. In this case, x = 5 found in some Andropogoneae is an aneuploid secondary derivative of a larger number (x = 9 or 10). The 20 chromosomes of maize (Zea mays L.) that were considered as a diploid appear as two duplicated sets of five chromosomes each (Moore et al. 1995). This most likely represents a secondary number derived by an alloploid event involving two closely related taxa with x = 5. This overall pattern implies chromosomal evolution via aneuploid reduction, followed by allopolyploidy that restored the ancestral number (x = ). Subfamilies Aristidoideae, Danthonioideae and Arundinoideae With Eriachne being basal to the second PACCAD clade, x = 10 appears ancestral (Fig. 2). The Aristidoideae, comprising Aristida, Stipagrostis and Sartidia, is basal to the remaining PACCAD (Soreng and Davis 1998; Hilu et al. 1999; Eriachne was not included in these studies) or sister to the Danthonioideae clade (Fig. 2; GPWG 2001). The basic chromosome numbers of x = 11 and x = have been reported for the subfamily (Tateoka 1962; Clayton and Renvoize 1986; Watson and Dallwitz 1992). In the Danthonioideae, x = 6, 7 and 9 were noted (Hunziker and Stebbins 1987; Watson and Dallwitz 1992). The Arundinoideae s.s. characteristically have x = 6, 9 and (Watson and Dallwitz 1992). Because x = 10 is not reported for these three subfamilies, aneuploidy in both directions from the ancestral number x = 10 seems to have played a role in their chromosomal evolution. Subfamily Chloridoideae As for the Panicoideae, the Chloridoideae have x = 9 and 10 as the common basic numbers, but the pattern is similar to that of the Paniceae where x = 9 is found in 86% of the species and x = 10 in 13% (dewet 1987). Chromosome numbers of x = 7 and 10 have been reported for Spartina (Reeder 1977; Gould and Shaw 1983; Watson and Dallwitz 1992) and Blepharidachne (Clayton and Renvoize 1986), x = 8 for Blepharoneuron, Erioneuron and Munroa, and x = for Sporobolus and a few Muhlenbergia (Clayton and Renvoize 1986). Unlike the Panicoideae, the basic number of x = 5 has not been reported for the Chloridoideae. The sister-group relationship between the arundinoid Centropodia and the Chloridoideae has been well established (Barker et al. 1995, 1999; Hilu et al. 1999; Hilu and Alice 2001). Centropodia has x = (Watson and Dallwitz 1992).

8 20 Australian Journal of Botany K. W. Hilu Parsimony analysis of matk-sequence data resulted in a polytomy at the base of the Chloridoideae that included Triraphis and two major clades (Hilu and Alice 2001). However, neighbour-joining analysis of that dataset and new molecular information (Neves, L. A. Alice and K. W. Hilu, unpubl. data) places Triraphis at the very base of the Chloridoideae, followed by a clade encompassing Uniolinae, Pappophoreae and Eragrostis. A basic chromosome number of x = 10 has been reported for Triraphis (dewet 1954), x = 9 or 10 for the Pappophoreae and x = 10 for all four genera of the Unioleae (Watson and Dallwitz 1992). Consequently, it is evident that aneuploid reduction from x = of Centropodia to x = 10 and 9 appeared early in the evolutionary history of the subfamily. Conclusions Mapping chromosome numbers on a robust phylogeny is a valuable approach that can provide strong evidence and important information on chromosome evolution. The proposed chromosomal-parsimony approach to assess chromosome evolution at various taxonomic levels should take into consideration the number of steps required to explain the changes, number of potential reversals, and extent of changes in chromosome number. This study has demonstrated that the ancestral chromosome number in the Poaceae appears to be x = 11. Aneuploidy from x = 11 to x = 9, followed by chromosome doubling, gave rise to the rediploidised paleopolyploid number x = of Anomochloa. Direct aneuploid shift from x = 11 to x = is less parsimonious and cytologically less feasible. Further aneuploidy from the ancestral number x = 11 resulted in x = found in Pharoideae and Puelioideae and ancestral basic chromosome numbers of remaining grass subfamilies. This finding is in partial agreement with the reduction hypothesis of Avdulov (1931), supported by Raven (1975), that proposed x = as the primitive number from which lower numbers originated by aneuploidy. In this case, x = 11, not x =, is the ancestral chromosome number. It is, however, in sharp discordance with Stebbins (1982, 1985) secondary polyploidy hypothesis and its modification by dewet (1987) which maintains that x = 11 and are secondary chromosome numbers derived from x = 5 and 6. Further chromosomal diversity observed in grasses was achieved via combinations of aneuploidy, euploidy and hybridisation. Aneuploid reduction can be achieved via chromosomal rearrangements and loss of the centromeric regions. Such events are quite deducible from genomic-mapping studies that used syntomy and circulisation approaches of grass genome (Devos et al. 1993; Moore et al. 1995). It is apparent from that approach that rearrangements of linkage groups and translocations among chromosomes, coupled with the loss of centromeric region, could result in reduction in chromosome numbers from the x = in rice to x = 7 in wheat and x = 5 in maize. 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